Effect of “push–pull” sensitizers with modified conjugation bridges on the performance of p-type dye-sensitized solar cells

Abstract

A series of “push–pull” sensitizers with modified conjugation bridges are designed and investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT), with the purpose of revealing the effect of different linker moieties on the performance of p-type dye-sensitized solar cells (DSSCs). Creatively, the electron-rich unit (thiophene) and the electron-deficient unit (pyrimidine) are studied as the linking groups in p-type sensitizers from a comparative perspective, two special bridge-sites and the lengths of conjugation bridges are also taken into account. Calculations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicate that there is efficient hole injection and dye regeneration for all the sensitizers. Importantly, the influence of the number of thiophene and pyrimidine moieties is seen mainly on the long-wavelength region and the short-wavelength region, respectively. According to the charge transfer properties and the driving forces of hole injection, dye regeneration and charge recombination (ΔG_inj, ΔG_reg and ΔG_CR, respectively), the increased length of the thiophene-based bridge close to the carboxyl group has a positive impact on the device performance. Likewise, for the pyrimidine-based bridges, it is probably the increased conjugation length between the donor and acceptor that significantly improves the device efficiency. Our intensive analysis on the π-bridges provides assistance for designing more efficient p-type photosensitizers, which contributes to the rational design for tandem DSSCs.

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1. Introduction

Plenty of attention has been paid to dye-sensitized solar cells (DSSCs), the environmentally-friendly, low-cost and easy-to-prepare photoelectrochemical devices, compared with traditional photovoltaic counterparts, since Grätzel and co-workers presented them in their seminal paper in 1991.¹ To date, extensive research on classical DSSCs (n-type DSSCs) varies from preparing new semiconductors,^2,3 designing and synthesizing new dyes^4,5 to exploring new electrolytic masses⁶ for the sake of perfecting the optical performance of devices and the power conversion efficiency (PCE) of batteries effectively. Nevertheless, there are limited investigations into the counterparts of n-type DSSCs, even though they play irreplaceable roles in inexpensive tandem devices (pn-type DSSCs).^7–10 Theoretically, the tandem DSSCs can overcome the Shockley–Queisser limit efficiently by absorbing both high energy photons and lower energy photons at the corresponding bandgap junction, therefore, this tandem arrangement can be perceived as a kind of executable strategy to enhance the overall efficiency. Moreover, they possess greater potentials with efficiencies up to 43% when compared with the simple n-type DSSCs whose theoretical efficiencies are 30%. The highest PCE on record for tandem DSSCs is 2.42%,¹¹ far below the 13% which is realized in n-type DSSCs cosensitized by zinc-porphyrin in a cobalt-based electrolyte.¹² One of the major obstacles in pn-type DSSCs with poor photovoltaic performance is the mismatch of photocurrent between the photoanode and the photocathode. Thus, it is worth exploring and promoting p-type DSSCs with improved current densities.

Since the first self-operating p-type DSSC was prepared using erythrosine by Lindquist and coworkers,¹³ plenty of potential sensitizers, including perylene, coumarin, porphyrin and their derivatives, have been explored in succession.^14–16 Upon the discovery of a new type of sensitizer with the “push–pull” configuration, 4-(bis-{4-[5-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl-amino)-benzoic acid (P1), the PCE reached 0.15% which is significantly higher than before.^17–19 Currently, with the application of abundant modification measures for improvement, a photocathode efficiency of 1.3% was gained by Spiccia and co-workers in p-type DSSCs.²⁰

Differing from the electron photoinjection into the conduction band of anode semiconductors such as TiO₂ in n-type DSSCs, for p-type DSSCs, the hole is injected into the inorganic semiconductor (NiO) from excited sensitizers followed by electron transferring to the redox couple in the electrolyte.^8,21–23 Plenty of studies have shown that the rapid back speed of the photoinjected hole from the p-type semiconductor to the reduced dye cuts down the separation efficiency of the charges, so as to limit the incident photon-to-current conversion efficiencies (IPCEs) and the resultant photocurrent.^24,25 One of the effective methods to improve the efficiency of the charge separation is to increase the distance of the charge transfer by employing longer bridge groups between the electron-withdrawing group (acceptor) and the anchoring group when the anchoring group attaches to the electron-donating group (donor).²⁶ Based on the typical “push–pull” sensitizer P1, Wu and coworkers²⁷ shed light on the effect of different π-bridges, indicating that 3,4-ethylenedioxythiophene was superior to thienyl and phenyl in terms of IPCE. Li and co-workers^26,28 synthesized and discussed the homologous sensitizer T1, in which an additional thiophene unit was inserted between the triphenylamine and carboxylic groups. They concluded that the performance of the device was improved significantly when the length of the bridge ligand near the anchoring group increases, while the increased conjugation bridge between the donor and acceptor had a negative effect on the device efficiency.

To further comprehend and demonstrate the effect of conjugation bridges of “push–pull” dyes on the device performance, a series of p-type sensitizers with modified conjugation bridges are designed and calculated. Herein, our primary interest in two kinds of bridges, one near the anchoring group (D-bridge) and another lying between the donor and acceptor (A-bridge), is to explore their effect on battery performance by employing electron-deficient pyrimidine and electron-rich thiophene, respectively. What’s more, the influence of their lengths is also taken into consideration. We believe that the better understanding of π-bridges, including the different electron-induced effects, position and length, could provide guidance for the further study of p-type sensitizers with higher PCEs.

2. Methods

2.1 Theoretical background

To the best of our knowledge, as the benchmark of solar cell performance for p-type DSSCs, the energy conversion efficiency (η) is closely correlated with three factors: the short-circuit photocurrent density (J_sc), the open-circuit photovoltage (V_oc) and the fill factor (FF), as follows:²⁹where I_s represents the total solar power incident on the cell.

As one of the important parameters in the theoretical study for DSSCs, J_sc can be evaluated by the following integral equation:^7,30,31

where LHE(λ) represents the light-harvesting efficiency at the specific wavelength, Φ_inj is the electron injection efficiency, and η_reg and η_coll denote the regeneration efficiency of the oxidized dye and the charge collection efficiency, respectively. In eqn (2), LHE(λ) is determined by the oscillator strength (f) of the absorbed dye molecule at the maximum wavelength λ_max. Φ_inj is associated with the driving force of hole injection (ΔG_inj) from the excited dye to the semiconductor. Furthermore, η_reg can also be measured by the driving force of regeneration (ΔG_reg) between the oxidized dye and the electrolyte. They are defined via the following expressions:^8,32

LHE = 1 − 10^−f (3)

ΔG_inj = e[E_VB(NiO) − (E_0–0 + E_red(dye))] (4)

ΔG_reg = e[E(I₂/I₃⁻) − E_red(dye)] (5)

V_oc, the other determining factor for evaluating the performance of DSSCs, can be expressed as follows:⁸

V_oc = E(I⁻/I₃⁻) − E_F(NiO) (6)

where E(I⁻/I₃⁻) and E_F(NiO) are the Fermi levels of the electrolyte iodine/iodide (I⁻/I₃⁻) and the semiconductor NiO. Notably, E_F(NiO) is closely linked to the hole injection and charge recombination processes in DSSCs.

Additionally, the process of unfavorable charge recombination can be evaluated by its corresponding free energies ΔG_CR, which can be expressed as:^8,32

ΔG_CR = e[E_red(dye) − E_VB(NiO)] (7)

where E_red(dye) is the reduction potential of the dye, and E_VB(NiO) represents the energy of the valence band of NiO.

The reorganization energy of the S1 excited state E_reorg(S1) is determined by the difference of E_S1(Q0) and E_S1(Q1):^33,34

E_reorg(S1) = E_S1(Q0) − E_S1(Q1) (8)

where E_S1(Q0) and E_S1(Q1) represent the energies of the S1 states corresponding to the equilibrium geometries Q0 and Q1, respectively. Based on eqn (8), E_0–0 can be calculated as follows:^33,34

E_0–0 = E_λmax − E_reorg(S1) (9)

where E_λmax represents the energy of the maximum absorption.

2.2 Computational details

Geometry optimizations of all the studied dye molecules in their ground states are performed on the basis of density functional theory (DFT). Several functionals (M062X, LC-WB97XD, PBE0, B3LYP) are applied to obtain the appropriate energy levels consistant with experiments (results shown in Table S1†), and it can be easily found that PBE0/6-31G(d) is enough for the present system. Thus, in this research, for all the isolated sensitizers (dyes) and their complexes linked with semiconductors (NiO/dyes), the hybrid functional PBE0 combined with the 6-31G(d) basis set is employed for C, H, O, N, S atoms, simultaneously, and the double-ξ (DZ) basis set LanL2DZ with corresponding pseudopotential is employed for Ni atoms. Taking the nickel oxide clusters into account, clusters NiO and (NiO)₉ are constructed in Materials Studio 7.0, and their structures are optimized at the DFT/PBE0/LanL2DZ level. Moreover, on the basis of the optimized ground-state geometries, the excited states of all the sensitizers are calculated using time-dependent density functional theory (TD-DFT); Boese and Martin’s τ-dependent hybrid functional (BMK) together with 6-31G(d) is used for their vertical electronic excitation energies and spectra simulation (calculations using other different functionals are listed in Table S2†). Additionally, the polarized continuum model (PCM) is adopted throughout, and acetonitrile is taken as the solvent in the whole investigation for evaluating solvent effects based on the experimental settings. What’s more, frequency calculations are used for all the geometry optimizations with the purpose of avoiding imaginary results. All the relevant calculations are carried out in the Gaussian 09 program package.³⁵

To measure the nature of the photoinduced electron–hole separation in depth, related electron densities of sensitizers are calculated using the code Multwfn 2.5.³⁶ Correspondingly, the distance of charge transfer and the fraction of charge exchange are calculated with DctViaCube.³⁷

3. Results and discussion

3.1 Molecular geometries and electronic properties

It is well known that π-bridges of dyes with “push–pull” structures have great influence on the light absorption, separation of the electron–hole pairs, charge transfer properties and so on.^38–42 For the purpose of investigating the influence of linker moieties on the IPCEs and structure–property relationships in p-type DSSCs, several sensitizers, employing two kinds of typical groups (pyrimidine units and thiophene units) with different electron-induced effects as π-linkers, are calculated and studied based on 5-(4-(bis(4-(5-(2,2-dicyano-vinyl)-thiophen-2-yl)phenyl)amino)-phenyl)thiophene-2-carboxylic acid (T1, also called T-D-1S-A-1S: other molecules are named in the form of T-D-xX-A-yY, where X is N or S, and Y is N or S; D and A represent the donor part and acceptor part, respectively; N and S denote pyrimidine and thiophene, respectively; x is 1, 2 or 3, y is 1, 2 or 3); their specific names and structures are presented in Fig. 1. Results of the optimized geometries indicate that there are certain discrepancies for electron-deficient groups (pyrimidine units) and electron-rich groups (thiophene units) in terms of the dihedral angle between the donor (triphenylamine, TPA) and the π-bridge. Corresponding with previous research, the dihedral angle between the phenyl in TPA and thiophene is approximately 21° when thiophene groups are taken as the linker moieties,³¹ which is ascribed to their slight steric hindrance of H atoms. However, for sensitizers with pyrimidine-based bridges, there are almost no angle torsion between pyrimidine and TPA. It is worth noting that the degree of conjugation and the physical properties are pertinent to the π-linker fragment. The difference in the degree of conjugation between thiophene units and pyrimidine units implies that pyrimidine-based dyes possess better electron delocalization due to the good planarity, however, the unfavorable conjugation of thiophene-based dyes may induce the localization of the frontier molecular orbitals (FMOs), leading to the efficient separation of hole–electron pairs, reducing their combination advantageously.³¹ In addition, different positions and lengths of π-bridges in dyes show little impact on structures’ torsion.

Fig. 1 Names and chemical structures of all the sensitizers.

Generally, the electronic excitation of dyes as well as their transition characters are closely correlated with their FMOs:^43,44 the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). As presented in Fig. 2, all the dyes show high LUMO energy levels for efficient electron transfer to the electrolyte (E(I₂/I₃⁻) = −4.15 eV),²⁶ and sufficiently low HOMO energy levels for efficient hole injection into the semiconductor NiO (E(NiO) = −5.04 eV).²⁶ Regarding the change of bridge length, calculations suggest that the energies of the HOMO and LUMO become higher slightly as the lengths of the bridges increase. On the one hand, thiophene-based bridges impact mainly on the HOMO energy level, and pyrimidine-based bridges near the acceptor have a great influence on both the HOMO and LUMO energy levels. However, on the other hand, energies of the HOMO and LUMO are almost unchanged when the D-bridge is a pyrimidine unit. Moreover, it can be speculated from the different energies between the HOMO and LUMO that the decreased band gaps realized in longer bridges are conducive to the red shift of the spectra.

Fig. 2 Schematic energy levels of the sensitizers, together with the valence band of NiO and I₂/I₃⁻ redox level. All calculations are obtained at the PBE0/6-31G(d) level.

To the best of our knowledge, for the ideal sensitizers which have fast hole injection from the excited state of the dye into the valence band of NiO, the HOMOs of the dyes should be predominantly delocalized over the anchoring groups and their surroundings, and the LUMO distributions of the dyes should extend onto the acceptor groups and keep away from the semiconductor with the purpose of reducing hole recombination.²⁶ All the distributions of HOMOs in our research systems are significantly located on the donor groups and their adjacent bridge-rings. As for the LUMOs, they are distributed mainly on the acceptor groups and their surroundings. Notably, with the lengthening of π-bridges, the distributions of LUMOs are increasingly far away from TPA, which is probably beneficial to the superior hole injection.

3.2 Optical properties

The absorption spectra of sensitizers are powerful tools repeatedly employed to evaluate their light-harvesting abilities. To investigate the optical properties of the dyes in our research system, the electronic spectra of isolated sensitizers and sensitizers linked with nickel oxide in acetonitrile are simulated and presented in Fig. 3. Apparently, two kinds of major absorption bands appearing in the short wavelength region (300–450 nm) and long wavelength region (500–650 nm) are gained in the absorption spectrogram, which are assigned to the B band and Q band, respectively.^34,45,46 Interrelated electronic transition data about the main absorption peaks in the B band and Q band are collected in Table S3.†

Fig. 3 Simulated absorption spectra obtained at the BMK/6-31G(d) level for all the dyes, NiO–dyes and (NiO)₉–dyes in acetonitrile solution with PBE0/6-31G(d) geometries.

It is noted from Fig. 3 that the absorption spectra of all the dyes are scarcely affected whether or not the sensitizers are linked with the semiconductor NiO. For the two systems in which the sensitizers are linked with nickel oxide (NiO–dyes and (NiO)₉–dyes, their optimized structures are presented in Fig. S1†), the spectral features can be simulated commendably, and their calculated spectra agree better with that of the isolated dyes. Based on further analysis of the spectra, the maximum wavelength of spectral absorption in the Q band appears red-shifted obviously as the lengths of the thiophene-based bridges directly linked with acceptor groups increase, from 498 nm (T-D-1S-A-1S), 527 nm (T-D-1S-A-2S) to 543 nm (T-D-1S-A-3S). In addition, the absorption strength also greatly improves, and the maximal oscillator strength of 3.3 is gained in T-D-1S-A-3S (from Table 1). Pyrimidine-based bridges close to acceptor groups have a significant influence on the B band. The absorption at 300–400 nm is enhanced dramatically as the lengths of the pyrimidine-based A-bridges increase. D-bridges, regardless of thiophene units or pyrimidine units, have little effect on the absorption spectra. With respect to the different π-bridges (thiophene-based bridges and pyrimidine-based bridges), the comparison of their absorption spectra implies that dye molecules with thiophene units as the conjugation bridges have a better ability to capture light.

Table 1 Estimated maximum wavelength of the spectral absorption (λ_max in nm), and reorganization energy (E_reorg(S1) in eV) of the first excited state (S1). f is the oscillator strength, LHE is the light-harvesting efficiency at maximum wavelength, and μ_T is the transition dipole moment from states S0 to S1. All calculations are obtained using the BMK functional with the 6-31G(d) basis set^a

Scheme	λmax (exp)	Ereorg(S1)	f	LHE	μT
a Data in parentheses are experimental values.
T-D-1S-A-1S	498/344 (488/369)	0.13	1.85/0.73	0.9859/0.8138	30.28
T-D-1S-A-2S	527/372	0.15	1.91/0.61	0.9877/0.7545	33.03
T-D-1S-A-3S	543/387	0.18	3.30/0.84	0.9995/0.8555	58.97
T-D-2S-A-1S	504/385	0.12	1.77/0.91	0.9830/0.8770	29.34
T-D-3S-A-1S	508/415	0.10	1.71/1.01	0.9805/0.9023	28.70
T-D-1S-A-1N	497/326	0.07	1.61/0.93	0.9755/0.8825	26.41
T-D-1S-A-2N	497/351	0.08	1.48/1.62	0.9669/0.9760	24.26
T-D-1S-A-3N	482/379	0.10	1.30/1.50	0.9499/0.9684	20.70
T-D-1N-A-1S	490/341	0.14	1.86/0.70	0.9862/0.8005	29.99
T-D-2N-A-1S	493/342	0.14	1.85/0.72	0.9859/0.8095	30.03
T-D-3N-A-1S	494/313	0.14	1.85/0.83	0.9859/0.8521	30.07

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To clarify the light absorption properties of the dyes visually and accurately, some key parameters are gathered in Table 1. It is well known that f reflects the light-harvesting efficiency (LHE) to a certain extent owing to the limited dye loading on the semiconductor film.¹⁸ As expressed in eqn (3), the stronger the f, the larger the LHE. According to the analysis of the Q band, the largest LHE of 0.9995 achieved in T-D-1S-A-3S is not prominent in comparison to the 0.9859 of T-D-1S-A-1S. In contrast, the difference of 0.35 in f leads to the change in LHE from 0.9859 (T-D-1S-A-1S) to 0.9499 (T-D-1S-A-3N). In summary, the LHE has no significant change for dyes in the Q band. With respect to the B band, the increase of oscillator strength to 1.62 is realized in T-D-1S-A-2N in comparison with 0.73 (T-D-1S-A-1S). Correspondingly, its LHE is improved to 0.976 compared with 0.8138 (T-D-1S-A-1S), which is conducive to the capture of sunlight in the short wavelength region.

As for the transition dipole moment from states S0 to S1 (μ_T), it is another way to confirm the ability of light-harvesting and electron-transition. As shown in Table 1, μ_T is enhanced in the order of T-D-1S-A-1S (30.28) < T-D-1S-A-2S (33.03) < T-D-1S-A-3S (58.97), T-D-1N-A-1S (29.99) < T-D-2N-A-1S (30.03) < T-D-3N-A-1S (30.07). This sequence shows that μ_T is dramatically increased with thiophene-based A-bridge lengthening, but for pyrimidine-based D-bridges, μ_T shows little change. Inversely, it decreases as T-D-1S-A-1S (30.28) > T-D-2S-A-1S (29.34) > T-D-3S-A-1S (28.70), T-D-1S-A-1N (26.41) > T-D-1S-A-2N (24.26) > T-D-1S-A-3N (20.70). This order reveals the conclusion that the longer the thiophene-based D-bridges, as well as pyrimidine-based A-bridges, the smaller the μ_T.

The reorganization energy of the S1 excited state E_reorg(S1) increases with the lengthening of A-bridge, exactly as T-D-1S-A-1S (0.13) < T-D-1S-A-2S (0.15) < T-D-1S-A-3S (0.18), T-D-1S-A-1N (0.07) < T-D-1S-A-2N (0.08) < T-D-1S-A-3N (0.10). But for the change of thiophene-based D-bridges, the smaller E_reorg(S1) is realized in T-D-3S-A-1S (0.10). An identical E_reorg(S1) (0.14) appears in the sensitizers with pyrimidine units as bridges.

3.3 Charge transfer properties

In general, for the “push–pull” dyes whose anchoring groups are located on the electron donor parts in p-type DSSCs, the electrons are transferred from the semiconductors to the donor parts of the sensitizers following the hole-injection through the conjugated D-bridges. In order to investigate some details of the charge transfer properties, including the electron transfer distance (L in Å), the fraction of electron exchange (Δe in |e⁻|), as well as the overlaps between the areas of density depletion and increment (Ω, isovalue: 4 × 10⁻⁴ e per au³), electron density difference plots of the electronic transition S0 → S1 for all the isolated dyes and dyes linked with nickel oxide (NiO–dyes and (NiO)₉–dyes) are calculated and listed in Tables 2 and S4,† respectively.

Table 2 Electron density difference plots of the electronic transition S0 → S1 for all the dyes performed in acetonitrile solvent using the BMK functional together with the 6-31G(d) basis set. L is the electron transfer distance (Å), Δe is the fraction of electron exchange (|e⁻|), Ω is the overlap between the regions of density depletion and increment (isovalue: 4 × 10⁻⁴ e per au³)^a

a Electron densities move from the green area to the purple area.
(a)
(b)
(c)
(d)

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From Table 2, the electron density depletion (green) localizes largely on the anchoring groups, conjugation spacers and donor parts, whereas the electron density increment (purple) mainly localizes on the acceptor segments, which contributes to the electron moving from the green area to the purple area. Additionally, the zones of the overlaps for the electron density depletion and increment occur in the A-bridges and their surroundings. Interestingly, the length of the electron transfer reflects the overlap between the regions of density depletion and increment to some degree. The longer the charge transfer distance, the fewer overlaps for the electron density; accordingly, it leads to a good charge separation. For the situation of (a) in Table 2, when the conjugation moieties in the A-bridges are two thiophenes, the Ω is only 0.1934, and the corresponding L reaches a length of 6.219. However, when the number of thiophenes is up to 3, the electron transfer distance reduces to 3.780, and the Ω becomes 0.3601. The results indicate that the longer thiophene-based bridges between the donor and acceptor do not correspond to better charge separation. There is probably a nonlinear change of charge distributions as the number of thiophenes increases, and the increased number of thiophene units in the A-bridges does not promote charge separation.^47,48 For (c), good charge separation is realized in different pyrimidine-based A-bridges, complying with the sequence of T-D-1S-A-1N (L = 4.515, Δe = 1.1681, Ω = 0.2726) < T-D-1S-A-2N (L = 6.261, Δe = 1.2914, Ω = 0.1264) < T-D-1S-A-3N (L = 8.023, Δe = 1.3276, Ω = 0.0522). This tendency implies that charge separation is enhanced significantly by lengthening the pyrimidine units in the A-bridges. Furthermore, changes in the D-bridges suggest that the electron transfer distance and the fraction of electron exchange can be improved dramatically for thiophene-based bridges, but are almost equal for pyrimidine-based bridges.

What’s more, for NiO–dyes and (NiO)₉–dyes presented in Table S4,† it can be clearly seen that there is similar behaviour in the charge transfer between dyes linked with nickel oxide and the isolated dyes. In the (NiO)₉–dyes, three molecules ((NiO)₉-T-D-1S-A-1S, (NiO)₉-T-D-2S-A-1S and (NiO)₉-T-D-2N-A-1S) have a special way of electron transfer. Their electron density depletion localizes partly on the nickel oxide, which indicates that their electron densities move from (NiO)₉ to the acceptor area.

3.4 Important performance parameters

Based on the equations discussed in the theoretical background, it is observed that sensitizers play an important role in the LHE, Φ_inj and η_reg during the photoelectric conversion process, so as to exert a great influence on the η through J_sc and V_oc.

The LHE has been evaluated by f in the section on optical properties. It is well established that the absorption spectra in the Q band are affected primarily by the electron-rich thiophene units, especially when they are located in the A-bridges. Conversely, the electronic-deficient pyrimidine groups exert a great influence on the absorption spectra in the B band, particularly when they are taken as the A-bridges. In addition, redox potentials of all the dyes are also calculated and analyzed, and the results are collected in Table 3. The electron injection driving force ΔG_inj, a key parameter associated with electron injection efficiency Φ_inj, is calculated on the basis of eqn (4). It is easily found that ΔG_inj is diminished a little as the lengths of the thiophene-based A-bridges increase. For the other situation, when the number of thiophene and pyrimidine moieties near the COOH group increases, the change of ΔG_inj is negligible. The −ΔG_inj is increased in the order 0.76 (T-D-1S-A-1N) < 0.88 (T-D-1S-A-2N) < 0.98 (T-D-1S-A-3N) as the number of pyrimidine units in A-bridges increases. Subsequently, the key regeneration process in DSSC devices has also been evaluated roughly through ΔG_reg, according to eqn (5). Correlated data of ΔG_reg in Table 3 elucidate that sensitizers (T-D-2S-A-1S and T-D-3S-A-1S) possess the greatest driving force of regeneration, 0.94 eV, compared with other dyes. Actually, previous literatures^49,50 have revealed that both electron injection and dye regeneration can occur efficiently when the driving force is larger than 0.2 eV. As a whole, all the sensitizers have effective injection and regeneration.

Table 3 Computed 0–0 transition energy (E_0–0 in eV), oxidized potential (E_ox(dye) in eV) and reduced potential (E_red(dye) in eV) of dyes in the ground state, ΔG_inj, ΔG_reg and ΔG_CR are the driving forces of hole injection, regeneration and recombination (eV), respectively. All calculations are obtained using the PBE0 functional with the 6-31G(d) basis set

Scheme	E0–0	Eox(dye)	Ered(dye)	ΔGinj	ΔGreg	ΔGCR
T-D-1S-A-1S	2.36	5.21	3.22	−0.54	−0.93	1.82
T-D-1S-A-2S	2.20	5.04	3.29	−0.45	−0.86	1.75
T-D-1S-A-3S	2.11	4.91	3.33	−0.40	−0.82	1.71
T-D-2S-A-1S	2.34	5.10	3.21	−0.51	−0.94	1.83
T-D-3S-A-1S	2.34	5.03	3.21	−0.51	−0.94	1.83
T-D-1S-A-1N	2.42	5.28	3.38	−0.76	−0.77	1.66
T-D-1S-A-2N	2.41	5.24	3.51	−0.88	−0.64	1.53
T-D-1S-A-3N	2.47	5.20	3.55	−0.98	−0.60	1.49
T-D-1N-A-1S	2.39	5.28	3.28	−0.63	−0.87	1.76
T-D-2N-A-1S	2.38	5.26	3.22	−0.56	−0.93	1.82
T-D-3N-A-1S	2.37	5.26	3.26	−0.59	−0.89	1.78

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Additionally, based on eqn (7), the process of unfavorable charge recombination is also discussed and analyzed. From Table 3, it is not difficult to find that all the ΔG_CR for dyes are positive, meaning that hindering the charge recombination is favored. For T-D-1S-A-1S, a great ΔG_CR of 1.82 is achieved, and as the number of thiophene moieties in the D-bridge increases, the ΔG_CR of 1.83 appears in T-D-2S-A-1S and T-D-3S-A-1S. With respect to the increase in lengths for A-bridges, including thiophene-based bridges and pyrimidine-based bridges, the ΔG_CR complies with the sequence of 1.82 (T-D-1S-A-1S) > 1.75 (T-D-1S-A-2S) > 1.71 (T-D-1S-A-3S), 1.66 (T-D-1S-A-1N) > 1.53 (T-D-1S-A-2N) > 1.49 (T-D-1S-A-3N). In summary, inserting thiophene groups into the D-bridge can adequately modify the sensitizers’ energy levels and lead to lower charge recombination. According to the above analysis of important performance parameters, corresponding tendencies are presented intuitively in Fig. S2.†

4. Conclusions

In conclusion, modified conjugation bridges of “push–pull” sensitizers play a vital role in the performance of p-type DSSCs. Two types of groups (thiophene and pyrimidine) with different electron-induced effects are discussed comparatively throughout; in addition, the locations of the bridges (A-bridge and D-bridge) as well as their lengths are also included in the calculation. Based on the geometries, dyes containing pyrimidine-based bridges exhibit greater planarity than dyes with thiophene-based bridges. It is valuable to note that all the designed sensitizers possess sufficient energies for hole injection and dye regeneration due to the HOMO being lower than the VB of NiO and the LUMO being higher than the redox potential of I₂/I₃⁻. Moreover, related exploration into the optical properties implies that there is a significant influence on the Q band as the lengths of the thiophene-based D-bridges increase. In contrast, increased lengths for pyrimidine-based A-bridges can lead to the strengthening of spectra in the B band. As for the analysis of the charge transfer properties, sensitizers with thiophene units as the D-bridges and pyrimidine units as the A-bridges exert longer charge transfer distances and more electron exchange as the lengths of the bridges increase.

Fundamentally and practically, the critical parameters (ΔG_inj, ΔG_reg and ΔG_CR) are closely associated with the performance of p-type DSSCs, particularly for the J_sc and charge recombination. Related analysis indicates that increased lengths of thiophene units in the D-bridges have little influence on ΔG_inj, ΔG_reg and ΔG_CR, but for the pyrimidine units in the A-bridges, they exhibit a favorable influence on the charge injection and recombination. Combined with previous research, we confirmed again that the increased lengths in thiophene-based bridges near the carboxyl group have a positive effect on the device performance. Similarly, for the pyrimidine-based bridges, it can be speculated that the increased conjugation lengths in the A-bridges could significantly improve the device efficiency. In addition, when comparing thiophene units with pyrimidine units, it is probable that pyrimidine units have a better contribution to the device efficiency, relatively, in our designed sensitizers. These results provide much guidance for the future design of p-type “push–pull” sensitizers with higher PCEs.

Acknowledgements

We acknowledge the generous financial support from Natural Science Foundation of China (21173169), Chongqing Municipal Natural Science Foundation (cstc2013jcyjA90015), and the Fundamental Research Funds for the Central Universities (No. XDJK2013A008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09263a

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